1. Field of the Invention
The present invention relates to a portable radio device such as portable information terminals, portable or cellular phones, and so on, and more particularly, to a folding, portable radio device having a pivotable antenna, in which the gain characteristic of the antenna is adjustable independent of the angle between the foldable packages around the specific axis.
2. Description of the Related Art
For portable radio devices such as portable information terminals, portable or cellular phones, and so on, antenna gain degradation is a serious problem that affects badly the communication performance. The first cause of this problem is the loss due to the fact that the maximum radiation orientation of the radiation pattern of an antenna does not accord with the orientation of incoming radio waves and/or the plane of polarization of an antenna does not accord with that of the incoming radio waves. The second cause of the same problem is the loss due to a human body
Concretely speaking, when portable radio devices of this type are used outside buildings or structures, the orientation of incoming radio waves is usually horizontal. Also, the plane of polarization of incoming radio waves is typically vertical, because vertical polarized radio waves has been usually used for transmitting operation as used in the Personal Digital Cellular (PDC) method to provide the digital automobile telephone and mobile telephone services. Thus, it is usual chat a half wavelength dipole antenna is used as the antenna of portable radio devices of this type, where a half wavelength dipole antenna has a radiation pattern like the number “8” that provides an advantage that the radiation pattern scarcely changes in horizontal directions. In this case, when portable radio devices of this type are in use, it is ideal that the antenna is held vertically to cope with vertical polarized radio waves.
Some of portable radio devices of this type are formed to be foldable to protect the operation buttons/switches and the display screen when they are not in use and are simply transferred. Conventionally, folding, portable radio devices of this type have a typical configuration that two packages are pivotably connected to each other with a hinge-shaped joint and that a telescopic whip antenna or a coil-shaped fixed antenna is mounted on one of the two packages. An example of this configuration is shown in FIG. 1.
In FIG. 1, the conventional folding, portable radio device 100, which is a portable information terminal, comprises a keyboard package 104 with a keyboard 106 and a screen package 103 with a display screen 107. These two packages 103 and 104 are pivotably connected to each other with a joint 105 like a hinge. A whip antenna 101 is usually attached to the screen package 103, because the antenna 101 is kept as high as possible when the device 100 is in use.
With a conventional folding, portable or mobile phone, there are provided with a transmitter package with a set of operation keys and a microphone, and a receiver package with a display screen and a speaker. These two packages are pivotably connected to each other with a hinge-shaped joint. A whip antenna is usually attached to the receiver package, because the antenna is kept as high as possible when the device is in use.
With the conventional folding, portable radio device (i.e., portable information terminal) 100 shown in FIG. 1, however, there is the following problem.
Specifically, when the device 100 is in use, it is usually held in such a way that the packages 103 and 104 are at an angle of approximately 180°. Thus, the antenna 101 is likely to be in an approximately horizontal position, resulting in gain degradation of the antenna 101.
FIG. 2 shows a graph showing the gain change of a half wavelength dipole antenna as a function of the tilt angle in a free space. The tilt angle of 0° means that the antenna is held vertically, where the gain is 0 dBd. The tilt angle of 90° means that the antenna is held horizontally. As seen from FIG. 2, the gain is decreased to −5 dBd and 10 dBd when the tilt angle is set at 60° and 90°, respectively.
Also, if the conventional device 100 is placed on a steel desk while the packages 103 and 104 are at an angle of an approximately 180°, the antenna 101 tends to contact or to be near the desk surface. In this state, the gain of the antenna 101 will decrease furthermore due to impedance mismatching.
Besides, with a conventional folding, portable or mobile phone, it is usually contacted with or located near the head of a human body during operation. In this case, a human body generates a shade of the phone for incoming electromagnetic waves and therefore, the radiation pattern will deform, degrading the antenna gain.
The reference character a in FIG. 2 indicates the decreased gain level when the antenna of the phone is placed near the head of a human body. Seeing the abscissa at the intersection of the gain curve with the horizontal line a, the state where the antenna is placed near the head of a human body corresponds to the state where the antenna is tilted at an angle of 60° from the vertical direction.
As explained above, with the conventional folding, portable radio devices, the antenna gain varies dependent on the angle between the packages connected pivotably and/or the location where the devices are used.